How to make a shade sail/Sun sail?
The cloth solution referenced by @batFINGER works for me:

Starting with the two (nearly) straight edges, first, Bridge Edge Loops with settings something like these:

.. and you're nearly there. A bit of Vertex &gt; Smooth-ing on the internal vertices might do. But if you want to, you can finish the stretch with a quick Cloth sim, as shown on the right, above, with settings very similar to those provided as 'Tent' in the referenced answer, and the straight edges pinned throughout, rather than just the corners.


(the sim runs to stability in about 30 frames, in the provided .blend)